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Application of soil biota in bioremediation of agrochemicals

Soil of agricultural field is a territory of several microorganisms, plants and animals. Each and every species of ecosystem contributes equally to maintain the functionality of this habitat. These biota also play an important role in eliminating toxic substances such as pesticides and chemical fertilizers from the fields. The biological remediation mainly includes microbial, animal and phytoremediation.

Microbial bioremediation

Microbial degradation of pesticide occurs through consumption of pesticide as a food source by bacteria or fungi. Each gram of soil contains millions of microbes. The use of microbial metabolic potential for eliminating soil pollutants has greater advantage over other commonly used physicochemical strategies. The remediation process by living organisms is earned out by two different

ways:

  • a) Biostimulation and
  • b) Bioaugmentation.

Biostimulation process stimulates the metabolic activity of indigenous microbial populations by the addition of specialized nutrients such as N or P, etc. and suitable physiological conditions which ultimately degrade the contaminants (Evans and Hedger 2001, Triudade et al. 2005). Biostimulation treatment could be necessary for the enzyme inducers and the co-metabolic substrates in the pesticide degradation pathways (Plangklang and Reungsang 2010). Bioaugmentatiou process is the introduction of exogenous microbes with specific catabolic abilities into the contaminated site to eliminate the toxic contaminants. It has been used to degr ade a wide range of chemical contaminants such as ammonia, hydrogen sulphide, and insecticides. Certain factors such as temperatures, pH levels, adequate soil moisture, aeration (oxygen) and amount of adsorption influence microbial degradation process. Adsorbed pesticides are slowly degraded as these are less available to some microorganisms.

Application of both biostimulatiou and bioaugmentation processes result in detoxification of toxic elements through biotransformation. Micro-organisms convert the toxic compounds into non-toxic compounds by the chemical reaction either aerobically or anaerobically. Anaerobic biotransfonnation is essentially used for degradation of organic compounds such as chlorinated hydrocarbons, polychlorinated phenols, and nitroaromatics, whereas aerobic biotransformation helps in transformation of thiocyanates, cyanates, aromatic hydrocarbons, gasoline monoaromatics, and methyl terf-butyl ether (Head and Oleszkiewich 2004, Perelo 2010).

Major species of bacteria that degrade the pesticides belong to genera Flavobacterium, Arthobacter, Aztobacter, Burkholderia, and Pseudomonas. The complete biodegradation of the pesticide involves the oxidation of the parent compound resulting in to carbon dioxide and water, which provides energy to microbes. To facilitate the degradation process of innate soil microbial population, certain additional pesticide degrading micro flora is recommended. Bioremediation of pesticides by microbes depends on the enzyme system but is also influenced by physical conditions such as temperature, pH and nutrients (Yao et al. 2015, Uqab et al. 2016). Kumar and Philip (2006) observed the biodegradation potential of three bacterial species such as Staphylococcus sp.. Bacillus circulans-l, and Bacillus circulans-ll to remediate pesticide eudosulfan by mixed culmre and pure culture.

Several fungal species like Flammulina velupites, Stereum hirsutum, Coriolus versicolor, Dichomitus squalens, Hypholoma fasciculare, Auricularia auricula, Pleurotus ostreatus, Avatha discolor and Agrocybe semiorbicularis help to degrade pesticides and release them into soil where it is susceptible to further degradation. These species have tremendous potential to remediate pesticides like triazine, phenylurea and chlorinated organophosphorous compounds. Repoits are available on bioremediation potential of white rot fungi Phanerochaete chrysosporium and Pleurotus pulmonarius to degrade highly recalcitrant pesticides like the chlorinated triazine herbicide 2-cliloro-4-ethylamine-6-isopropylamino-l,3,4-triaziue (atrazine), HCH, dieldriu, diuron, aldriu, DDT, etc. Phanerochaete chrysosporium has been observed to degrade a number of toxic xenobiotics such as aromatic hydr ocarbons (Benzo alpha pyrene, Phenantlirene, Pyrene), chlorinated organics (Alkyl halide msecticides, Chloroanilines, DDT, Pentaclilorophenols, Trichloroplieuol, Polychlorinated biphenyls, Trichlorophenoxyacetic acid), nitrogen aromatics (2,4-Dinitrotoluene, 2,4,6-Trinitrotoluene-TNT) and several miscellaneous compounds such as sulfonated azodyes (Yao et al. 2015).

The microorganisms used in bioremediation are either natural or genetically modified, known as “super strains”. These engineered micro-organisms are developed to introduce the efficient catabolic genes to stimulate the degradation capacity in indigenous microorganisms. Microbes with their catabolic gene and enzymes have been isolated and identified by several workers to demonstrate the degradation of different pesticides like carbofuran, carbarlyl, baygon or aldicarb (Singh 2008).

Enzymes actively take part in the degradation of pesticide compounds in environment, via biodegradation by soil microorganisms. The first bacterium was isolated front a soil sample of Philippines and identified as Flarobacteriuni sp. ATCC 27551 to degrade OP (organophosphate) compounds. Certain fungal enzymes, especially oxidoreductases, laccase and peroxidases, have potential to remove the polyarornatic hydrocarbons (PAHs) contaminants in terrestrial ecosystems (Watanabe et al. 2000, Uqab et al. 2016).

 
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